1. Field of the Invention
The present invention relates to a selective reduction catalyst that dissolves nitrogen oxide contained in an exhaust gas from a diesel engine by the reaction of nitrogen oxide with urea liquid and to an engine exhaust gas purifier using it.
2. Description of the Related Art
As an exhaust gas purifier for reducing nitrogen oxide (hereinafter, referred to as “NOx”) contained in an exhaust gas from a diesel engine, there have been known an exhaust gas purifier, wherein a selective reduction catalyst is provided in the middle of an exhaust passage of a diesel engine, and a liquid injection nozzle which can inject urea liquid toward the selective reduction catalyst is provided at an exhaust pipe on the upstream side of the selective reduction catalyst (See Patent Document 1, for example). The conventional selective reduction catalyst comprises a catalyst support having a plurality of through holes formed in parallel with each other and an active component having a catalytic action and carried by partition walls of the catalyst support. And in the conventional diesel-engine exhaust gas purifier provided with this selective reduction catalyst, the urea liquid injected from the liquid injection nozzle is heated by heat of the exhaust gas and hydrolyzed, and ammonia is generated. And the ammonia serves as a reducing agent for purifying NOx in the exhaust gas by the selective reduction catalyst so that the NOx emitted to the atmosphere can be reduced.
On the other hand, in the exhaust purifier provided with the selective reduction catalyst in the middle of the exhaust passage in this way, in order to prevent surplus ammonia from being emitted directly into the atmosphere, an ammonia oxidation catalyst for oxidizing the surplus ammonia is usually provided separately from the selective reduction catalyst at the exhaust passage on the downstream side of the selective reduction catalyst. And the ammonia oxidation catalyst oxides the surplus ammonia that has passed through the selective reduction catalyst, not serving as a reducing agent, so that the ammonia can be prevented from being emitted directly into the atmosphere.